Electrophotographic material, element and method

ABSTRACT

Electrophotographic material which is sensitive to white light and is bichargeable, i.e., capable of accepting electrostatic charges of either polarity. The material comprises a mixture of photoconductive zinc oxide and mercuric sulfide, in an insulating binder, the zinc oxide in the mixture being in an effective concentration not in excess of about 48 percent by volume of the mixture.

United States Patent Logue Jan. 21, 1975 [54] ELECTROPHOTOGRAPHIC MATERIAL, 3,658,523 4/1972 Noe 252/501 x ELEMENT AND METHOD FOREIGN PATENTS OR APPLICATIONS [75] Invent Daniel L08, Gabriel, Calif- 639,318 4/1963 Canada 96/].8

[73] Assignee: Bell & Howell Co., Chicago, Ill.

Primary Examiner-Roland E. Martin, Jr. [22] Ffled' 1973 Attorney, Agent, or Firm-Nilsson, Robbins, Bissell, [21] Appl. No.: 321,432 Dalgarn & Berliner [52] US. Cl. 96/l.8, 96/15 [57] ABSTRACT [51] Int. Cl G03g 5/08 Electro photographic materlal WhlCl'l 1s sens1t1ve to Fleld of Search and is bichargeable, capable of accept:

ing electrostatic charges of either polarity. The mate- [56] References cued rial comprises a mixture of photoconductive zinc UNITED STATES PATENTS oxide and mercuric sulfide, in an insulating binder, the 2,937,944 5/1960 Van Dorn et al 96/ 1.8 zinc oxide in the mixture being in an effective concen- 3,060,l34 10/1962 Elder et al. 96/1.8 X tration not in excess of about 48 percent by volume of Jones the mixture 3,378,371 4/1968 Jarvis.... 3,634,333 l/l972 Tamai 252/501 3 Claims, 7 Drawing Figures ELECTROPHOTOGRAPHIC MATERIAL, ELEMENT AND METHOD FIELD OF THE INVENTION The present invention generally relates to electrophotographic materials and more particularly to bichargeable electrophotographic materials of novel composition and to recording elements and a method of producing a fixed visible image utilizing the same.

BACKGROUND AND SUMMARY OF THE INVENTION Various types of photoconductive coatings have been provided for xerographic or electrophotographic recording and copying. These are generally inorganic photoconductive particles or coatings which can accept a charge in the dark which is dissipated when exposed to certain wavelengths of light. n-Type potoconductors accept a negative charge in the dark, but do not accept a positive charge and the opposite effect is realized with p-type photoconductors. The charge remains on the photoconductive surface until discharged in those areas exposed to light or other impinging radiation having a wavelength to which the photoconductor is sensitive. Whether the photoconductor is of the por n-type, an electrostatic image, i.e., a charge pattern corresponding to the exposure image pattern, can be formed on the photoconductive surface by applying the discharging radiation in the form of an image. This latent image can be developed to a visible image corresponding to the original image pattern by contact with colored toner particles having a charge opposite in plarity to that of the latent image. With such a procedure, a positive print is obtained from a positive original, and a negative print is obtained from a negative original.

Zinc oxide is one of the most widely used photoconductive materials. In its pure state it is n-type, accepting a negative charge but almost no positive charge. The material can be discharged by ultraviolet or deep blue light but it is relatively insensitive to discharge by visible light. Various dye sensitizers have been incorporated to extend the light sensitivity of zinc oxide and other techniques have been used. In U.S. Pat. No. 2,937,944, zinc oxide particles are combined with red hexagonal mercuric sulfide, in a ratio of 1:1 to 30:1 (zinc oxidezmercuric sulfide) and dispersed in a resin binder to provide a coating having panchromatic response.

In many instances it is desirable to provide a positive print from a negative, and several systems for accomplishing such result have been devised. Usually, these systems are subject to certain disadvantages. In one system a developer of the same charge as the electrostatic image may be used for positive printing from negative originals. A developer comprising charged toner particles of the same polarity as that of the electrostatic charge image is employed so that the charged toner particles deposit on the charge-free areas of the photoconductive surface and are repelled from the chargebearing areas. While the technique is effective, care must be taken to keep the two types of toner particles separated. Otherwise, blurring of the usable image and spotting of the image-free background on the positive print may occur. Usually, this means that separate machines must be employed, rendering the procedure expensive and cumbersome.

In another system, after the electrostatic charge image is formed from the negative on the photoconductive surface, it is reversed by subjecting it to a greater electrical field of opposite polarity to that of the electrostatic image to cancel out the charge from the originally charged areas and impose a charge only in the originally charge-free areas. Thereafter, the newly charged areas are developed with toner. Obviously, this system is complicated, requires close and careful control of field potentials and other variables and utilizes expensive equipment.

Bichargeable photoconductive materials, i.e., chargeable equally to either polarity, do exist, but usually are deficient in one or more characteristics or are relatively expensive and difficult to manufacture. Thus, while pure zinc oxide is an n-type photoconductor, it can be carefully treated with hydrogen sulfide to form a surface having molecules of electrondeficient zinc sulfide to exhibit bichargeability; see for example US. Pat. No. 3,060,134. The procedure is expensive and uniformity of the product and results obtained therefrom may vary according to firing parameters.

Various other substances exhibit bichargeability, such as selenium, cadmium sulfide and zinc sulfide phosphers, but these materials provide better results when charged to one polarity rather than the other. Moreover, many bichargeable substances are not sensitive to white light, that is, are not sensitive over the entire visible light spectrum but are sensitive only to certain wavelengths requiring the use of dye sensitizers to achieve panchromatic response.

Accordingly, there is a need for a simple bichargeable electrophotographic material which is sensitive to white light and which provides good xerographic results with charges of either polarity.

The present invention provides an improved, white light-sensitive, bichargeable photoconductor for xerographic systems. The photoconductor is easily prepared, has a long shelf life and provides good results in producing visible positive prints when charged to either polarity. The material comprises a mixture of zinc oxide and mercuric sulfide in certain specified proportions dispersed in particulate form in an electrically insulating binder on a conductive base to form the improved recording element of the invention. Over the range of preparations set forth more particularly hereafter, the electrophotographic material exhibits both bichargeability and does not require dye sensitization for white light sensitivity. The mixture is substantially uniformly dispersed in particulate form within an electrically insulating binder, such as a suitable plastic resin. Preferably the binder is in a weight ratio to the mixture of about 1:5-l0. An improved recording element is also provided and comprises the described mixture and a binder on a conductive base. The element is used in the formation of a visible image in the improved manner of the present invention, as hereinafter set forth in more detail.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an enlarged schematic cross-section of a recording element in accordance with the present invention;

FIG. 2 schematically depicts the step of charging the element of FIG. 1, for the production of a positive print from a positive;

FIG. 2A schematically depicts the step of charging the element of FIG. .1 for the production of a positive print from a negative;

FIG. 3 schematically depicts latent image formation on the element of FIG. 2;

.FIG. 3A schematically depicts latent image formation on the element of FIG. 2A;

FIG. 4 schematically depicts development of the latent electrostatic image of FIG. 3 to obtain a positive print; and

FIG. 4A schematically depicts development of the latent electrostatic image of FIG. 3A to obtain a positive print, i.e., image reversal.

DETAILED DESCRIPTION The drawings illustrate the present method but are not necessarily drawn to scale nor do they represent actual quantities, e.g., volume thickness, size, etc.

Referring to FIG. 1, a recording element 10in accordance with the present invention is provided which comprises an electrically grounded conductive supporting base 12 and a photoconductive coating layer 14. The conductive base 12 can be any suitable conventional material such as conductive paper, metal, foil or plate or the like, which is utilized for such purposes. If the coating layer 14 is disposed on an insulative layer such as non-conductive paper (not shown) that latter layer is disposed on the conductive base 12. For the purposes of the present invention, therefore, by conductive base is meant an electrically conductive component or the like to which coating 14 is directly applied, or to which a support for the coating 14 is applied.

The coating non-dye-sensitiz ed mercuric sulfide l8, preferablyred hexagonal mercuric sulfide. The volume ratio of zinc oxide to mercuric sulfide is from 10:90 to 48:52. The most preferred mixture formulation is about 40 volume percent zinc oxide and about 60 volume percent mercuric sulfide, at which optimal results, including discharge speed, are obtained. The particle sizes of the zinc oxide and mercuric sulfide are not critical but effect resolution of the mixture, and appropriate particle sizes are known to the art. Generally the particles are of about 0.2 to about 3.0 microns or finer.

The mixture is substantially uniformly dispersed within an electrically insulating binder 20, such as polystyrene, epoxy resins, silicon resin, acrylic polyester methacrylic polyester, polyvinyl chloride, polyvinyl acetate or the like to form coating 14. Preferably, the photoconductive mixture is present in a weight ratio to the binder of about l0:l, although the exact ratio is not critical so long sufficientamounts of the mixture of zinc oxide 16 and mercuric sulfide 18 are present to accomplish the intended purposes.

Referring to FIGS. 2 and 2A, as in a conventional electrophotographic process, the recording element is electrostatically charged in the dark through the use of a charging device, such as a corona discharge device 22. Thus, a suitable conventional corona discharge device 22 is used to impart an electrostatic charge (of negative polarity in the case of FIG. 2 and positive polarity in the case of FIG. 3) to thesurface of the photoconductive coating 14. Referring to FIGS. 3 and 3A, a latent electrostatic image is then formed on the charged surface of the coating 14 by exposing it to a layer 14 comprises an intimate mixture of fine particles of non-dye sensitized zinc oxide 16 and I 4. l y light image, for example, by'passing'light from a source 24 through a photographic negative 26.'In areas where the light strikes, the electrostatic charge is discharged,

leaving the remaining electrostatic charge in a latent image pattern 28 or 28A. The pattern 28 in FIG. 3, is composed of negative electrostatic charges and the pattern 28A, in FIG. 3A, is composed of positive electrostatic charges.

Development of the latent image pattern 28 or 28A is accomplished by any conventional method, using either a liquid or dry developer containing electrostatically charged toner particles, all as known to the art. The same developer is used for both the pattern 28 of FIG. 3 and 28A of FIG. 3A. Referring to FIGS. 4 and 4A, the developer contains positively electrostatic charged colored toner particles 30. As shown in FIG.

4, particles 30 deposit directly on image 28 areas due to attraction to the negatively charged image so as to provide a visible image 32. As shown in FIG. 4A, particles 30 are repelled by the'positively charged image 28A and deposit on the image-free portion of the surface bearing the image 28A to provide a reversed visible image 32A.

Thus, with the recording element of the present invention, either negative or positive electrostatic charges are accepted by the layer 14 and therefore, either a positive or negative (reversal) copy can be obtained from the same original using the same developer. The visible images 32 and 32A can be fixed by conventional means, such as by evaporationof liquid supporting medium, when a liquid developer is employed, or by heating of dry developer 30 to fuse the resin binder used therewith.

It will be understood that the same positive print can be obtained by the described novel method from an original positive or original negative, employing the procedures exemplified by FIGS. 1 through 4 andby FIGS 1A through 4A. It will be further understood that developer containing negatively charged toner particles can be employed in place of the developer exemplified in the described drawings, but with opposite mode results, and thatthe same developer can be used for both of the procedures described. Accordingly, the present-method of forming a visible fixed image has great flexibility and can be carried out in a single simple apparatus at low cost and with rapidity, due to the novel recording element and its novel photoconductive mixture. Further features of the invention are set forth in the following specific examples, wherein all parts are by weight unless otherwise specified.

EXAMPLE 1 To approximately 25 parts of percent solution of a non-conductive binder, comprising a modified acrylic in a solvent comprising an equal mixture of toluene and xylene there is added about 40 grams of photoconductive zinc oxide in particulate form having an'average particle size of about 0.3 microns. 88.5 parts of GP. grade red mercuric sulfide, having an average particle size of about 2.5 microns, is added to the binder-zinc oxide-solvent mixture to provide a volume ratio of zinc oxide to mercuric sulfide of about 40:60 and a weight ratio of binder to zinc oxide plus mercuric sulfide of about 1:8. The resulting mix is agitated for about 5 minutes in a Waring blendor to a I-Iegman reading of about 6 and then coated on a conductive base paper to provide, when dried, a thickness of about 1.0 mil.

The thus-formed recording element is then tested in two parallel tests as follows:

In the first test, the recording element is charged by passing it under a corona discharge device set at +6KV as read on a Monroe brand electrometer. The charged recording element is then exposed to white light while the discharge occurring is monitored on the electrometer. The light source is a monochromator equipped with a xenon light and no grating. The light during the exposure is piped to the recording element surface through a fiber optic tube. The photoconductive coating is found to accept +250 volts and is discharged by approximately 65 foot candle seconds of white light.

EXAMPLE 2 The same photoconductive coating, conditions and parameters as set forth in Example 1 are used, except that the photoconductive coating is charged using a 6KV corona device. The charged coating is found to accept 230 volts and is discharged by approximately 55 foot candle seconds of white light.

The above two tests illustrate that when a recording element is composed of the novel mixture of the invention combined with an electrically insulating resin, it can be utilized with substantially equal efficiency when electrostatically charged with either polarity, that is, it is substantially equally sensitive in either mode to white light. The novel photoconductive mixture of the invention exhibits rapid discharge on the order of about 0.1 second on exposure to white light. The mixture is photosensitive over substantially the entire visible spectrum. Thus, the coating is able to function efficiently as a bichargeable photoconductor, eliminating the need for two developers employing toner particles of opposite polarity in order to develop positive prints from positive originals and negative originals. Moreover, it eliminates complicated equipment and procedures.

Accordingly, an improved method of reproducing a fixed visible image is provided. The method is rapid, simply conducted and highly flexible, utilizing the novel bichargeable photoconductive mixture of the invention in a recording element. Various modifications, changes, alterations and additions can be made in the present bichargeable photoconductive mixture and the recording element employing the same. All such changes, modifications, alterations and additions as are within the scope of the appended claims which form part of the present invention.

1 claim:

1. An improved electrophotographic recording element capable of accepting electrostatic charges of either polarity, said element comprising a conductive base layer, and a layer of electrophotographic material disposed on said base layer and comprising a mixture of photoconductive zinc oxide, and red, hexagonal mercuric sulfide, the volume ratio of said zinc oxide to said mercuric sulfide being about 40:60, said mixture being dispersed in particulate from in an electrically insulating binder, said binder being in a weight ratio to said mixture of about 1:5 to about 1:10.

2. The improved electrophotographic recording element of claim 1 wherein said binder comprises a resinous binder and wherein said mixture is uniformly dispersed therein in fine particulate form.

3. The improved electrophotographic recording element of claim 1 wherein said volume ratio of said zinc oxide and said mercuric sulfide is about 40:60.

* l =l l= g;;g UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, ,9 Dated Januar 21, 1975 lnventofls) Daniel R. Logue It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 10, after "present" add -method, its steps and parameters and in the present.

Column 6, line 2 4, change from to -form----.

' a Signed and Sealed this thirteenth D 3y 0 f April 1 9 76 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (unmxissiom'r of Pan-1'1 Is and Tradenmrks 

2. The improved electrophotographic recording element of claim 1 wherein said binder comprises a resinous binder and wherein said mixture is uniformly dispersed therein in fine particulate form.
 3. The improved electrophotographic recording element of claim 1 wherein said volume ratio of said zinc oxide and said mercuric sulfide is about 40:60. 